1. Field of the Invention
The present invention relates to a detection device for measuring force and pressure, provided with a monomode optical fiber, into which polarized light is fed and at the output of the fiber is received by a detection arrangement sensitive to the polarization of the light, the influence of the force or the pressure to be measured being substantially perpendicular to the longitudinal axis of the optical fiber and the detection arrangement being provided with means for analyzing the light in accordance with at least two different directions of polarization, together forming an angle comprised between 30.degree. and 60.degree..
2. Description of Related Art
Detection devices of the type referred to above are based on the photo-elastic effect in a monomode fiber, i.e. the rotation of the polarization or the change in the ellipticity of the polarization in response to stress. These devices are used more specifically in intrusion detection, supervising and safety systems. The fiber is, for example, buried into the ground, following a perimetrical line outside buildings, to detect the presence of intruders even before they reach the buildings.
In the prior art, two devices arranged at 45.degree. with respect to each other, were proposed to be arranged at the output of the fiber. Such a device is disclosed in EP-A-0,120,999 (FIG. 8 and the entire page 11). The reason for such an arrangement is that the device described in said document uses either a special fiber or a special treatment by twisting the fiber around its axis, in this case at the rate of 15-20 turns/meter. This twist must not change versus time, which would lead to false alarms and, furthermore, in the case of an intrusion on a place covered by one or more turns, would cause problems because a desensitization by compensation may occur.
It is furthermore interesting to conceive a system which functions with conventional monomode fibers. When such fibers are burried in the ground, the transmission of the stress load cannot be controlled perfectly and two types of phenomena may be observed without it being certain in advance which one will occur (moreover, there could be a combination of these two phenomena).
These two physical effects are:
a rotation of the polarization plane, for example, by torsion of the fiber.
a change in the state of the polarization obtained by compression, for example a linear polarization then becoming elliptical, this effect being moreover very frequent: it is denoted birefringence through compression.
Thus, the transmission of a polarized light, injected at the input of the fiber and "analysed" at the output, is changed under the action of an intrusion.
Parasitic effect (imperfect fiber, thermal or mechanic effects) may modify the orientation and the shape of the polarization, both in space and in time, which might result in a very strong attenuation of the signal, commonly denoted "fading" by the expert.